Rheostat



Nov. 18, 1952 P. vLAHos RHEOSTAT Filed March 2, 1951 2 SHEETS- SHEET 1 a W? ff QL; Q

IN VEN TOR.

T [3l/MMM@ Nov. 18, 1952 Filed MarGh 2, 1951 P. VLAHOS RHEOSTAT 2 SHEETS-SHEET 2 P5 reo EAA/0.9, NVENTOR.

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Patented Nov. 18, 1952 RHEOSTAT Petro Vlahos, Culver City, Calif., assignor to Mction Picture Research Council, Inc., Hollywood, Calif., a corporation of California Application March 2, 1951, Serial No. 213,506

(Cl. Ztl- 48) 'i Claims.

This invention is concerned generally with means for varying the electrical impedance in a circuit, whereby, for example, the current flowing in the circuit, or the voltage between two points of the circuit, may be variably controlled.

The invention is of particular utility in connection with circuits carrying direct current, the variable impedance then being ordinarily essentially resistive in nature. For that reason, and for simplicity and clarity of expression, the iollowing description will be directed primarily to direct current rheostats. However, it will be understood that the invention may be utilized in alternating as well as direct current circuits, and that the resistors of any such rheostat may be replaced by other types of electrical impedances, such for example as inductances and capacitors.

The invention is concerned more particularly with the typical problem of providing a relatively heavy duty rheostat in which the resistance elements may conveniently be spaced from the control mechanism, in which the tendency to arcing at the control contacts is reduced or eliminated, and in which the wire bundle required to interconnect the control mechanism and the resistance elements is much smaller than has previously been required for similar service.

That typical problem is solved in accordance with the present invention by providing an ordered array of resistor elements, switching means for bringing those elements successively into a circuit in parallel, and means for progressively increasing the current through each resistor as it is brought into the circuit.

A full and clear understanding of the inventicn and of its further objects and advantages will be had from the following description of a specic illustrative embodiment, which is not intended to be limitative on the scope of the invention. The following appended drawings form a part of that description:

Fig. l is a diagram illustrating in schematic form a preferred embodiment of the invention in open circuit position;

Fig. 2 is a schematic diagram like Fig. 1, but showing the rheostat in an intermediate position of adjustment;

Fig. 3 is a schematic diagram 0I the circuit associated with Fig. 2;

Fig. 4 is an axial view, partly broken away, showing an illustrative rheostat switch in accordance with the invention;

Fig. 5 is a section on line 5-5 of Fig. 4;

Figs. 6 and 7 are schematic diagrams of previous types of rheostat; and

Fig. 8 is a graph illustrating the general form of the relationship between direct current and voltage in a circuit which may be opened slowly without arcing.

In Figs. l and 2, illustrative primary and secondary switch elements l2 and I4 are mounted for relative rotative movement on a rigid support or frame, indicated schematically at I0. Switch element l2 comprises a circular contactcarrying member I5 of insulative material, rotatively xed on a shaft I8, which carries also driving means such as pulley 26 and is journaled in frame l0 in any suitable manner, not shown in detail. Member I6 carries a number ci ordered primary switch contacts C, shown as Ci, C2, C3 CN, the exact number of such contacts depending upon the conditions of service for which the apparatus is intended. The number N of primary contacts may, for example, be fourteen. As illustrated, contacts C are spaced uniformly along the periphery of member I6, but any suitable arrangement and type of mounting may be used that provides the proper type of progressive engagement with the secondary contacts to be described.

Primary resistors RI, R2, R3, RN have one terminal of each connected in parallel via line il to ons main terminal T of the rheostat. The other terminals of resistors R are connected respectively to the corresponding primary switch contacts CI, C2, C3, CN by means of wires Wl, W2, W3, WN, which may be iiexible over at least a portion of their length to permit limited rotational movement of switch element l2. In the drawing the intermediate portions of wires W are omitted for clarity of representation.

Secondary switch element I-l comprises, as an illustration, an arcuate support 28 of insulative material, mounted rigidly on frame l@ and carrying a number of ordered secondary switch contacts c, shown as cl, c2, c3 cn. The total number n of secondary switch contacts depends, as does N, upon detailed design considerations. As an example, n may be twenty-two. Contacts c, as illustrated, are spaced along the inner circular edge of member 26 in position to make electrical contact with primary contacts C as switch element I2 is rotated. Secondary contacts c are preferably relatively closely spaced, and each contact except cn is of relatively small angular extent about shaft I8. The last one, cn, of the secondary contacts is of relatively great angular extent, or is otherwise so arranged as to be capable of simultaneously engaging all, or substantially all, of the primary contacts C.

A number of secondary resistors r (equal to one less than the number of secondary contacts) are connected between the respective pairs of consecutive secondary contacts by means of secondary wires w. The central portions of those wires are omitted in the drawings for clarity of illustration. Thus, resistor rl is connected by wires wl and wZ between cI and c2; r2 is connected by w2 and w3 between c2 and c3, etc. The last secondary resistory is designated as H11-I) and is connected between the last secondary contact en and the next preceding contact c(n-I) by conductors wn and 10m-I The second main terminal t of the rheo'stat may be connected directly to secondary contact cn by a line 27. As is clear from the figures, connections w of the secondary resistors r may be entirely independent of the main terminals T and t. Alternatively, wire wn from one terminal of secondary resistor TOL-l) may be convnected directly to main terminal t, line 2'] then Vremote control for theV switching means is afforded by a handwheel 30, having the form of a pulley and carrying a handle 32, which is rotatably mounted in any convenient position with relation to frame'l. K The endless belt 34 represents an illustrative type of driving linkage between handwheel 3) and shaft I8. It will be understood that either or both of the switch elementsl may be movable with respect to frame I E), and that 'their relative movement may be translational rather than rotary. The present showing of one xed and one rotatable switch element is merely illustrative.

With the Vswitch in the position shown in Fig. 1, there is no electrical Contact between primary contacts C and secondary contacts c. Hence the circuit between terminals T and t is open. By clockwise rotation of switch element I2 (as shown in Figs. 1 and 2) with respect to element It, primary contact CI iirst engagessecondary contact cl, closing a circuit between T and t via lead I'I,'primary resistor RI, wire WI, the said engaged contacts, wire wi, all cf lthe secondary resistors r in series and lead 2l. Because of the connection of rl, r2 MTL-I) in series with RI, a relatively small current is thereby allowed to now between T and t.

As switch element I2 is further rotated clockwise, primary contact CI engages successively the. ordered array of Vcontacts c, preferably engaging each contact before leaving the preceding one, eliminating progressively from the circuit one after another V,of lthe secondary resistors TI, 12, and so on. YThe resistance in series with RI is thereby progressively decreased, and the current between T and t is correspondingly increased. When primary contact CI reaches the last of the secondary contacts cn, the primary resistor RI is inserted directlyacross T and t. During any further clockwise switch rotation, CI maintains contact with the circularly elongated contact cn, and RI remains in the circuit.

Primary contact 'C2 is similarly moved, by clockwise switch rotation, progressively across the ordered array of secondary contacts, rst inserting R2 into the circuit in series with all or rl, r2 rm-I), and then progressively removing one after another of the secondary resistances from the circuit. When C2 ultimately reaches cn, R2 and RI are connected in parallel between T and t, and that connection is maintained dur-- ing further switch rotation. The same type of action takes place successively with respect to the remaining contacts C.

As an illustration, when the control switch is in the typical intermediate position shown in Fig. 2, terminals T and t are connected by a resistance network of the type illustrated schematically in Fig. 3. Primary resistors RI and R2 are connected directly in the circuit in parallel, while R3 is connected in parallel with them but in series with secondary resistors ri through rl3. Similarly, R4 is parallel connected, but with its current limited by series connection with r6 through TIS; and R5 is similarly in series with rE-Tli and T5413. At the particular switch position of Fig. 2, primary resistors R6, Rl RN remain entirely out of the circuit, while TI and T5 are momentarily shorted out by primary contacts C5 and C4, respectively.

control while maintaining the relatively large current carrying .capacity that results from the use of discrete switch contacts of relatively large area.

Substantially the same'smoothness of operation can be obtained with a more compactfcrm of switch byspacing adjacent primary contacts more closely than the total space lbetween cI and cn. For example, as shown in Fig. 2, more `than one primary contact may engage steps of the secondary rheostat at the same time. When that is thecase, the total number of control steps -available is 'at least N11; divided'by the greatest number of primary contacts that may simultaneously engage steps of the secondary rheostat (not counting cn). 'If the intervalA between adjacent primary contacts C is so related to the interval between adjacent-secondary contacts c that twoprimary contacts do not simultaneously make or break their engagement with' respective secondary contacts (for example if thefsaid two intervals are incommensurable), then the total number of control steps may approach, or even equal, Nn, even for the more compact type of arrangement illustrated.

Indexing means maybe provided in any'usual mannerif it is desired to limit the rotarymovement of the switchA to whatever discrete control steps may be made available 4by the particular detailed arrangement of the various contacts. However, the described rheostat is particularly well suitedV to operation without such indexing means, as will be described.

Because` of 'practical considerations, well understood in the art, the Vactual mechanical mounting and arrangement 'of 'the contacts `is preferably different from thatshown'schematically in Figs. 1 and 2. For'exarnple, in'a preferred mechanical' arrangernent,A the primary and secondary contacts may all lie at the same radius from the axis of 'shaft' I8, 'and theirworking faces may lie in a commonplane normal to that axis, rather thanina cylindrical surfacealsjindicated in Figs. l and VV2. 'Any Ysuitable 'resilient `means may be employed to provide Substantially uniform contact pressure between engaged contacts. All primary contacts, land all secondary contacts, need not be of the same type; and some or all of the contacts may include more than one contact element.

Figs. l and 5 show a typical specific embodiment of a switching means which illustrates a few of the great variety of physical features that may be included in an embodiment of the invention. As shown, a cup-shaped member 50 of insulative material supports secondary contacts cl, c2 cm-l) in apertures 52 in its flat base portion 54, in which they fit movably and are urged upwardly (as seen in Fig. 5) by individual springs 56. Those springs may also connect the respective contacts to wires w which connect to secondary resistors r as already described. Secondary contact cn is embedded in the inner face of cylindrical rim 58, extending nearly half way around that rim, as shown in Fig. 4. The relative placement of the contacts cl through cca-I) at different radii from the center and their progressive relative angular displacement in a clockwise direction are shown clearly in Fig. 4.

The primary contacts C are mounted on a disk Si] of insulative material, fixed on shaft 62, which is journalled axially in member 5U as by a bushing 66. Any suitable means, not shown, may be provided for rotating disk 60 with respect to member 50. Each of the primary switch contacts C comprises two distinct contact elements, a radially elongated metal insert E8 embedded in disk @Ei flush with its inner face in position to engage successively the ordered array of secondary contacts cl to cuz-I) as disk G is rotated clockwise; and a metal shoe 'l0 resiliently mounted on the outer face of disk 60 as by a spring 'I2 in position to engage secondary contact cn upon further clockwise rotation of disk 60. Springs 12 may provide electrical connection between the respective shoes 1E) and the corresponding inserts GB. Connection to primary resistors R is made by respective wires W.

As illustrated, the leading edges of both elements 68 and 'l0 of each secondary contact C lie in the same axial plane. However, their angular relation may be arbitrary, so long as the angular relation between the leading edges of secondary contacts CUL-l) and on corresponds, that correspondence being such that, on clockwise rotation of disk 60, element engages cn shortly after element B8 has engaged cm-I The angular spacing between consecutive primary contacts C (angle p in Fig. 4) is preferably enough greater than the angle between the leading edge of secondary contact cI and the trailing edge of cut-I) that one primary contact element leaves CUL-I) before the next one engages cl. Then only one primary contact is on the secondary rheostat at a time, and it is immaterial that several secondary resistors may be snorted by a primary contact. As illustrated, the secondary contacts cl through 3m-l) become progressively larger. That is not necessary, but may be advantageous, since the current that those contacts are called upon to carry increases progressively with the number of the contact.

One of the limiting factors in the service capacity of an impedance varying device employing discrete control steps is the tendency to arcing at the contacts as the latter are opened. That is true in both alternating and direct current service, but is particularly troublesome in connection with direct current rheostats. One

method of solving that problem is to provide a very large number of discrete control steps, so that the change in current and voltage at each step is too small to sustain an arc. In power rheostats of previously available types, it is entirely impracticable to provide a large enough number of discrete control steps for that pur-A pose. That is particularly true when, as is` ordinarily the case, the resistor elements must be located at a relatively great distance from the control switching mechanism. With a conventional series type rheostat, shown typically in schematic form in Fig. 6, not only line |21, but also each of the connecting wires Wa through We must be of sufficient capacity to carry the entire current flowing in the circuit at the time that wire is in use. To provide a large number of contacts with such a system requires a correspondingly large number of wires, each of large size, which is not ordinarily practicable.

A parallel type controller avoids a part of that difficulty. As illustrated in Fig. '7, only lines II'I and |21 are called upon to carry the full circuit current. Each of the wires Wa through We need carry only that portion of the current passing through its associated resistor. Particularly when a relatively large number of steps is provided, the current through each resistor is ordinarily a small fraction of the total. However, the required wire bundle is still impracticably large if suflicient steps are provided to prevent arcing.

In accordance with the present invention, the values of the various resistors R and 1 may be selected in any appropriate manner, depending upon the service for which the rheostat is intended. Particularly when the resistors and control switch are to be relatively distantly spaced, so that reduction of the size of the bundle of connecting wires is a primary factor, it is preferred to select the values of the various primary resistors R in such a, Way that each carries approximately the same maximum current. The maximum current through each primary resistor flows when its primary contact has just reached the last secondary contact cn as the total current is being increased, or just before it leaves cn as the total current is being decreased. If the rheostat is to be connected in series with a substantially constant Voltage source and a substantially constant impedance load, the effective voltage across the terminals of the rheostat tends to decrease as the current in the circuit is increased. Under such conditions, the primary resistors preferably decrease successively in resistance value approximately fast enough to compensate for the decreasing applied voltage as they successively reach contact cn, so that each will carry substantially the same maximum current. Detailed values for the respective re sistances can be calculated from the'constant-s of the circuit, preferably selecting the most severe conditions that may arise. For example, if the circuit includes a motor, the resistance of the motor when stalled may be used as the load impedance.

Values of the various secondary resistors are preferably so selected that the tendency to arc as each secondary contact is opened is kept substantially constant over all of the contacts. A great practical advantage of the invention is that it is ordinarily practicable to keep the values of current and voltage at each contact low enough to avoid arcing even as the contacts are opened slowly. A graph may be plotted from acreage pointsjobtained experimentally, showing values `of'current and voltage at which a'slowly opened pair of` contacts will just fail to Vmaintain an arc. A ygraph of that type is shown illustratively in Fig. 8. Under the particular experimental conditions represented by that graph, an arc Vis maintained across opening contacts only if the values of the current carried by the contacts vbefore opening and of the voltage across the 4contacts after opening correspond to Aa point 'of the graph above the line VI. Starting with the llast secondary resistor, denoted by Ir(1bl) in Figs. 1 and 2, Aand working back toward ri, vit is possible to compute successively, from such a graph'as Fig. 8 and vfrom vthe other constants'of the circuit in which the device is to be used, the lmaximum value that can be assigned to each resistor without causing arcing at its contact is opened.

In the accompanying table are givenill'ustrative values for primary and secondary resistors vthat were determined in the manner indicated for a rheostat to be used for controlling a circuit including a source of 120 volts D. C. and a motor having a resistance of about 0.85 ohm when stalled. The calculation assumes primary contacts sumciently widely Yspaced that only one at a time can engage second contacts cl to cm-i).

Primary Resistors Secondary Resistors Number OhmsV Number Ohms (gg) l 5. 4 21 .46 .46 2 4. 6 20 57 l. O3 3 3.9 19 58 l. 61 4 3. 4 18 65 2. 26 5 3. 0 17 77 3. 03 6 2. 7 16 87 3. 9 7 2. 5 y 15 1. 0 4. 9 8 2. 3 14 1. 1 6. 0 9 2. 1 13 l. 3 7.3 10 1. 8 l2 1. 5 8. S l1 1. 4 1l 1:8 l0. 6 12 1. o 10 2. c y12'. t 13 7 9 2. 5 15. 1 14 5 8 2. 8 17:9 7 3. 4 21. 3 6 4.0 25.3 5 4. 9 30. 2 4 6. 0 36. 2 3 7. O 43. 2 2 8. 8 52.0 1 11. 0 63. 0

With the values given, there is a Ypossibility of arcing as the control is moved to fully vopen position, that is, as contacts Ci, cl are opened. That can be avoided by connecting a 200 ohm resistor across those contacts, so that-the'hi'ghest available resistance value between T and t is 63 ohms plus 200 ohms instead of in'ni'ty A1- ternatively, two or three additional secondary resistance steps may be provided.

It will be understood that the invention mayl intervals for the secondary array, a set of lprimary impedances connected in parallel 'between the respective contacts of the primary arrayiand the other terminal, and a set of secondary impedances connected between the several pairs of consecutively engaged contacts of the secondary array. t

2. Means for varying the electrical impedance between two terminals, the said means comprisingswitching meansincluding primary and secondary ordered arrays of mutually spaced and insulated contacts, the said arrays being mounted for relative movement such that the contacts of the primary array are successively engageable in ordered progression with the successive contacts of the secondary array until lall contacts of the primary array simultaneously engage only the last engaged contact of the secondary array. one terminal being connected to the said last engaged contact of the secondary array, aset of primary impedances of progressively increasing value spaced from the switching means and connected in parallel between the respective contacts of the primary array and the other terminal, the values oi the successive primary impedances Vbeing such that each primary impedance and its connection to its contact will carry substantially the same current upon rst engaging the last engaged contact of the secondary array and means connecting the said last engaged contact of the Vsecondary array to the respective other contacts yof the secondary array through respective total impedances that decrease progressively for the successive contacts inthe direction of their said successive engagement.

3. Means for varying the electrical impedance between two terminals, the said means comprisingprimary and secondary ordered arrays of mutually spaced and insulated switching contacts, means for movingone array .bodily with respect to the other to produce successive engagement of each of the respective contacts of the primary array in ordered lprogression with the successive contacts of the secondary array,

the last engaged contact of the secondary array being connected to vone terminal and'being sumciently elongated in the direction of the said movement to engage simultaneously all of the contacts of the rst array, a set of primary impedances connected in parallel between the respective contacts of the primary-array and the other` terminal, anda set of secondary impedances connected between the several pairs of consecutively engaged contacts of the secondary array,

-the values of the successively engaged secondary iicient, by approximately the same margin, to

maintain an arc. y Y

4. Means for varying the'electrical impedance between two terminals in a circuit having predetermined characteristics, the said means comprising primary and secondary ordered arrays of mutually spaced and insulated switching contacts, means for reversibly moving one array bodily with respect to the other to produce successive engagement of each of the respective contacts of the primaryarray in ordered progression with the successive contacts of the secondary array, the last engaged contact of the secondary array being connected to one terminal and being suciently elongated in the direction of the said movement to engage simultaneously all of the contacts of the rst array, a set of primary impedances of progressively increasing value connected between the respective contacts of the primary array and the other terminal, and a set of secondary impedances of progressively decreasing value connected between the several pairs of consecutively engaged contacts of the secondary array, the Values of the successive primary impedances being such that each primary impedance will carry substantially the same current upon rst engaging the last engaged contact of the secondary array, and the values of the successive secondary impedances .being such that, as the first engaged contact'of the primary array is disengaged by reversed movement from each of the contacts of the secondary array, the current and voltage in each circuit that is thereby opened are insufficient, by approximately the same margin, to maintain an arc.

PETRO VLAHO Si.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

